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A growth trial lasting for 12 weeks was conducted in a semi flow-through system to determine the dietary potassium requirement of juvenile grass carp (Ctenopharyngodon idellus) with an initial body weight of 4.8 g. Seven isonitrogenous and isoenergetic semi-purified diets were compounded with different dietary potassium levels: 1. 50, 2.32, 3.28, 5.30, 7.12, 9.45 and 10.44 g kg −1 diet. The results showed that specific growth rate and feed efficiency of grass carp were significantly (P b 0.05) affected by dietary potassium level, the highest value of specific growth rate was in 5.30 g kg −1 group and feed efficiency values in 5.30, 7.12 and 9.45 g kg −1 groups were significantly higher than those in the other groups. The survival rate of grass carp in the present study was not affected by dietary potassium level (P N 0.05). The Na + -K + ATPase activity value in the gills varied significantly (P b 0.05) with dietary potassium level and showed an increasing and then decreasing trend, and the highest value was in the 7.12 g kg −1 group. Dietary potassium level showed significant (P b 0.05) effect on serum SOD, ALP and AST activities. SOD and ALP activities increased and then decreased as potassium level increased, while AST decreased first and then increased. Serum TP and TC contents increased and then decreased with potassium level (P b 0.05). TG, ALT and BUN were not affected by dietary potassium level (P N 0.05). Body potassium and moisture contents increased and lipid content decreased with dietary potassium content, while ash content in fish body increased first and then decreased as potassium content increased (P b 0.05). Body protein in the 10.44 g kg −1 group was significantly lowered (P b 0.05). However, calcium, magnesium, phosphorus and sodium contents in dry matter were not affected by dietary potassium level. When the potassium content of water was 1.86-3.10 mg L −1 , judging from the polynomial regression analysis for the specific growth rate of fish and the Na + -K + ATPase activity in the gill, and the broken-line analysis for body potassium content, the optimal dietary potassium content for juvenile grass carp was 4.65, 7.27 and 5.98 g kg −1 diet respectively.

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Gamma aminobutyric acid (GABA) is one of the inhibitory neurotransmitters that may have the ability to regulate the appetite. This study aimed to evaluate the effects of dietary GABA, as a feed additive, on growth performance and antioxidant status of juvenile grass carp. Five isonitrogenous and isoenergetic experimental diets with graded levels of GABA (0, 20, 50, 100 and 200 mg/kg) were fed to grass carp juveniles (mean weight: 2.0 g) for 8 wk. The results showed that the specific growth rate (SGR) increased significantly with dietary GABA level up to the 100 mg/kg diet, and then decreased in the 200 mg/kg group (P < 0.05). The difference in SGR value between the 50 and 100 mg/kg groups was not statistically significant (P > 0.05). The survival rate was not significantly affected by dietary GABA level (P > 0.05). No significant difference was observed in the whole body composition among treatments (P > 0.05). The hepatopancreatic glutathione peroxidase (GPx) activity increased significantly (P < 0.05) with the GABA level, peaked in the group with GABA 100 mg/kg, and then decreased. The hepatopancreatic superoxide dismutase and total antioxidant capacity activities showed the same trend as GPx. Inversely, the hepatopancreatic malondialdehyde content decreased first and then increased with the dietary GABA level (P < 0.05). No significant differences were observed in the gene expression of neuropeptide Y (NPY), cholecystokinin, leptin, and ghrelin in the hepatopancreas among groups. However, the NPY and ghrelin mRNA expression levels in the brain increased significantly with the GABA level, peaked in the groups of 50 mg/kg, and then showed a decreasing trend. On the basis of quadratic regression analysis of SGR, the optimum content of dietary GABA in juvenile grass carp is suggested to be a 87.5 mg/kg diet.